The Solar System Portal
and planets of the solar system (distances not to scale)
The Solar System is the gravitationally bound system of the Sun and the objects that orbit it, either directly or indirectly. Of the objects that orbit the Sun directly, the largest are the eight planets, with the remainder being smaller objects, such as the five dwarf planets and small Solar System bodies. Of the objects that orbit the Sun indirectly—the moons—two are larger than the smallest planet, Mercury.
The Solar System formed 4.6 billion years ago from the gravitational collapse of a giant interstellar molecular cloud. The vast majority of the system's mass is in the Sun, with the majority of the remaining mass contained in Jupiter. The four smaller inner planets, Mercury, Venus, Earth and Mars, are terrestrial planets, being primarily composed of rock and metal. The four outer planets are giant planets, being substantially more massive than the terrestrials. The two largest, Jupiter and Saturn, are gas giants, being composed mainly of hydrogen and helium; the two outermost planets, Uranus and Neptune, are ice giants, being composed mostly of substances with relatively high melting points compared with hydrogen and helium, called volatiles, such as water, ammonia and methane. All eight planets have almost circular orbits that lie within a nearly flat disc called the ecliptic.
The Solar System also contains smaller objects. The asteroid belt, which lies between the orbits of Mars and Jupiter, mostly contains objects composed, like the terrestrial planets, of rock and metal. Beyond Neptune's orbit lie the Kuiper belt and scattered disc, which are populations of trans-Neptunian objects composed mostly of ices, and beyond them a newly discovered population of sednoids. Within these populations are several dozen to possibly tens of thousands of objects large enough that they have been rounded by their own gravity. Such objects are categorized as dwarf planets. Identified dwarf planets include the asteroid Ceres and the trans-Neptunian objects Pluto and Eris. In addition to these two regions, various other small-body populations, including comets, centaurs and interplanetary dust clouds, freely travel between regions. Six of the planets, at least four of the dwarf planets, and many of the smaller bodies are orbited by natural satellites, usually termed "moons" after the Moon. Each of the outer planets is encircled by planetary rings of dust and other small objects.
The solar wind, a stream of charged particles flowing outwards from the Sun, creates a bubble-like region in the interstellar medium known as the heliosphere. The heliopause is the point at which pressure from the solar wind is equal to the opposing pressure of the interstellar medium; it extends out to the edge of the scattered disc. The Oort cloud, which is thought to be the source for long-period comets, may also exist at a distance roughly a thousand times further than the heliosphere. The Solar System is located in the Orion Arm, 26,000 light-years from the center of the Milky Way galaxy.
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is a moon of the planet Jupiter
, discovered in 1610 by Galileo Galilei
. It is the third-largest moon
in the Solar System
and the second largest in the Jovian system, after Ganymede
. It is not a part of the orbital resonance
that affects three inner Galilean satellites—Io
and Ganymede—and thus does not experience appreciable tidal heating
. Callisto rotates synchronously
with its orbital period, so the same hemisphere is always turned toward Jupiter. It is composed of approximately equal amounts of rock
, with a mean density
of about 1.83 g/cm3
. Compounds detected spectroscopically
on the surface include water ice
, carbon dioxide
, and organic compounds
. Investigation by the Galileo
spacecraft revealed that Callisto may have only partially differentiated interior covered by a thick icy crust and possibly a subsurface ocean of liquid water at depths greater than 100 km. Prominent surface features include multi-ring structures, variously shaped impact craters
, and chains of craters and associated scarps
, ridges and deposits. Callisto is surrounded by an extremely thin atmosphere
composed of carbon dioxide
and probably molecular oxygen
, as well as by a rather dense ionosphere
Selected picture -
Comet Hale–Bopp sails across the sky in the vicinity of Pazin in Istria, Croatia. To the lower right of the comet the Andromeda Galaxy is also faintly visible. The comet was visible to the naked eye for a record 18 months, twice as long as the Great Comet of 1811. At perihelion, it shone brighter than any star in the sky except Sirius, and its two tails stretched 30-40 degrees across the sky. The passage of Hale-Bopp was notable also for inciting a degree of panic about comets not seen for decades. Rumours that the comet was being followed by an alien spacecraft gained remarkable currency, and inspired a mass suicide among followers of the Heaven's Gate cult.
Andreas Cellarius's illustration of the Copernican system, from the Harmonia Macrocosmica (1660)
Schematic of the hypothetical Oort cloud, with a spherical outer cloud and a disc-shaped inner cloud
Neptune and its moon Triton, taken by Voyager 2. Triton's orbit will eventually take it within Neptune's Roche limit, tearing it apart and possibly forming a new ring system.
The Ring nebula, a planetary nebula similar to what the Sun will become
Beyond the heliosphere is the interstellar medium, consisting of various clouds of gases. The Solar System currently moves through the Local Interstellar Cloud.
Location of the Solar System within the Milky Way
Meteor Crater in Arizona. Created 50,000 years ago by an impactor about 50 metres (160 ft) across, it shows that the accretion of the Solar System is not over.
Hubble image of protoplanetary discs in the Orion Nebula, a light-years-wide "stellar nursery" probably very similar to the primordial nebula from which the Sun formed
Relative size of the Sun as it is now (inset) compared to its estimated future size as a red giant
The geology of 2014 MU69
("Ultima Thule"), the first undisturbed planetesimal
visited by a spacecraft, with comet 67P
to scale. Notable surface features are highlighted at right. The eight subunits of the larger lobe, labeled ma
, are thought to have been its building blocks. The two lobes came together later, forming a contact binary
. Objects such as MU69
are believed in turn to have formed protoplanets
Orrery showing the motions of the inner four planets. The small spheres represent the position of each planet on every Julian day, beginning July 6, 2018 (aphelion) and ending January 3, 2019 (perihelion).
Orrery showing the motions of the outer four planets. The small spheres represent the position of each planet on every 100 Julian days, beginning January 21, 2023 (Jovian perihelion) and ending December 2, 2034 (Jovian perihelion).
Ceres – map of gravity fields: red is high; blue, low.
Simulation showing outer planets and Kuiper belt:
a) Before Jupiter/Saturn 2:1 resonance
b) Scattering of Kuiper belt objects into the Solar System after the orbital shift of Neptune
c) After ejection of Kuiper belt bodies by Jupiter
Orbit of Jupiter
Orbit of Saturn
Orbit of Uranus
Orbit of Neptune
Zooming out the Solar System:
- inner Solar System and Jupiter
- outer Solar System and Pluto
- orbit of Sedna (detached object)
- inner part of the Oort Cloud
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